VSE not playing in realtime (not caching)?
While you're playing your sequence, you should see a little fps counter in one corner of the preview. If it's red and below your frame rate you set beforehand, than your computer can't handle the preview rendering in real time. This can be related to memory, but (at least from my experience) it more likely depend on your CPU.

What you can do to improve the playback, is setting up proxy files. They're typically smaller so playback is easier.